1. Field of the Invention
The present invention relates to a magnetoresistive element applied to a nonvolatile memory or the like, and an MRAM using the magnetoresistive element.
2. Related Background Art
Recently, magnetic memory elements for storing information by using a magneto-resistance effect receive attention as high-density, high-response, nonvolatile solid-state memory elements. MRAMs (Magnetic Random Access Memories) attract attention as memory devices using magnetic memory elements. The magnetic memory element stores information by the magnetization direction of a magnetic layer, and can constitute a nonvolatile memory for semipermanently holding information. MRAMs are expected to be used as various memories such as information storage elements for a portable terminal and card. Especially an MRAM with a magnetic memory element using a spin tunneling magneto-resistance (TMR) effect can utilize a high-output characteristic obtained by the TMR effect. This MRAM also allows high-speed read, and its practical use is expected.
A magnetic memory element may have a structure comprised of a memory layer and reference layer. In general, the reference layer is a magnetic material layer whose magnetization direction is fixed or pinned in a specific direction. The memory layer is a layer for storing information, and is generally a magnetic layer capable of changing its magnetization direction by externally applying a magnetic field. The logic state of the memory element is determined by whether the magnetization direction within the memory layer is parallel to that within the reference layer. If these magnetization directions are parallel to each other because of the MR (Magneto-Resistance) effect, the resistance of the magnetic memory cell decreases, if these directions are not parallel, i.e., are antiparallel, the resistance of the memory cell increases. The logic state of the memory cell is determined by measuring its resistance.
Information is written in the memory cell of the MRAM by changing the magnetization direction within the memory layer by a magnetic field generated by flowing a current through a conductor. Written information is read out using an absolute detection method of detecting the absolute value of a resistance or a differential detection method of changing the magnetization direction of a detection layer in read.
The memory element must shrink in feature size for high integration degrees. Generally in a longitudinal magnetization layer, the spin curls at the film edge due to a demagnetizing field within the film surface along with the miniaturization. The memory element cannot stably store magnetic information. To prevent this problem, the present inventor has disclosed in U.S. Pat. No. 6,219,275 an MR element using a magnetic film (perpendicular magnetization film) magnetized perpendicular to the film surface. The perpendicular magnetization film is free from any curling even upon miniaturization. When this film is used as a memory element, it can sufficiently shrink in feature size to increase the density of the MRAM.
The MR element includes two magnetic layers stacked via a nonmagnetic layer. A stray magnetic field leaked from one magnetic layer is applied to the other magnetic layer. The magnetic field is kept applied even in the absence of an external magnetic field.
FIGS. 14A and 14B show examples of the magnetization direction of a TMR element having a perpendicular magnetization film. A magnetic film 100 and a magnetic film 200 higher in coercive force than the magnetic film 100 are stacked via a nonmagnetic film 300. In both the examples shown in FIGS. 14A and 14B, the magnetic film 200 is magnetized downward. The magnetic film 100 is magnetized downward in FIG. 14A, and upward in FIG. 14B. The magnetic film 200 is a pinned layer always magnetized downward. “0” is recorded in the state of FIG. 14A, and “1” is recorded in the state of FIG. 14B.
FIG. 15A shows the MH curve of this element (graph showing the relationship between the magnetization and an application magnetic field) on the assumption that no stray magnetic field is leaked from the magnetic film with a squareness ratio of 1. Since the magnetization direction of the magnetic film (pinned layer) 200 cannot be changed, the resistance changes in correspondence with the magnetization direction of the magnetic film (memory layer) 100. In the absence of an offset magnetic field, information can be recorded in the memory layer only by applying a magnetic field H1 or H2 equal to a coercive force Hc. The magnetic field H1 switches the magnetic film 100 from the upward direction to the downward direction. The magnetic field H2 switches the magnetic film 100 from the downward direction to the upward direction.
In practice, the magnetic film (pinned layer) 200 applies a downward magnetic field to the magnetic film (memory layer) 100. The MR curve shifts by an offset magnetic field, as shown in FIG. 15B. In this case, the recording magnetic field is H2=Hc+Ho and H1=Hc−Ho. The magnetic field necessary to change the state of FIG. 14B to that of FIG. 14A decreases by Ho. To the contrary, the magnetic field necessary to change the state of FIG. 14A to that of FIG. 14B increases by Ho. FIG. 15B shows the magnetization curve at this time. This graph shows that the current value flowing through a write line changes depending on a rewrite magnetization direction. The current consumption increases, or when the current exceeds the allowable current density of write line wiring, write fails. In this case, the intensity of a switching magnetic field changes depending on information recorded on a memory cell. If memory cell information which requires the switching magnetic field H2 is rewritten in recording information in memory cells arrayed in a matrix via two perpendicular write lines, adjacent memory cell information which requires the switching magnetic field H1 is also rewritten. Such erroneous recording operation may occur at a high possibility. If the offset magnetic field Ho becomes larger than the coercive force Hc, as shown in FIG. 15C, only one resistance value can be taken in zero magnetic field. This makes absolute detection difficult.
When the squareness ratio is not 1, as shown in FIGS. 16A and 16B, a resistance value M2 in zero magnetic field becomes smaller than a maximum magnetization value Mmax of an antiparallel magnetization state. The resistance value also changes depending on the magnetization intensity of the low-coercive-force layer. In this case, a readout resistance value difference M2−M1 decreases, degrading the detection sensitivity. A memory element with a squareness ratio of not 1 is more greatly influenced by the offset magnetic field. This phenomenon occurs even in an offset magnetic field Ho smaller than the coercive force Hc. Note that M1 represents the minimum resistance value in the absence of an external magnetic field; and M2, the maximum resistance value in the absence of an external magnetic field. FIG. 16A shows the resistance value in the presence of the offset magnetic field Ho, and FIG. 16B shows the resistance value in the absence of the offset magnetic field Ho.
For a squareness ratio of not 1, even application of a magnetic field equal in intensity to the coercive force does not completely saturate the magnetization, as shown in FIG. 16B. A magnetic field which completely saturates the magnetization, M=Ms, will be called a magnetization saturation magnetic field Hs. When the memory layer completely saturates to be antiparallel to the pinned layer, the resistance value maximizes to a constant value with respect to the magnetic field. That is, the magnetic field which saturates in the resistance value is equal to Hs, as shown in FIG. 16B. For a squareness ratio of 1, the coercive force can be regarded equal to a magnetization switching magnetic field. For a squareness ratio of not 1, the coercive force cannot be regarded equal to this magnetic field. In this case, the magnetization must be switched by applying a magnetic field larger than that having a squareness ratio of 1. In the presence of an offset magnetic field generated by a stray magnetic field, the difference in the intensity of a magnetic field applied to switch the magnetization becomes larger between a direction in which the magnetization is easy to switch and a direction in which the magnetization is difficult to switch. If such an element is employed as the memory element of an MRAM, the above-described erroneous operation may occur at a higher possibility. Malfunction may occur when a magnetization switching magnetic field is not controlled in the use of a magnetoresistive element as the memory element of an MRAM.
It is an object of the present invention to solve the problem that a static magnetic field from one magnetic layer offsets the switching magnetic field of the other magnetic layer in a magnetoresistive element used as a memory element or the like, and to provide a memory element using this magnetoresistive element and its recording/reproduction method.